1. Field of the Invention
The present invention relates to a radiation therapy apparatus used for a therapy of diseases such as malignant tumors, and particularly to the radiation therapy apparatus including a multi-leaf collimator device which allows an extent of an object that is exposed to radiation (which will be referred to as an “irradiation field” hereafter) to be set with high precision.
2. Description of the Related Art
From the perspective of radiation protection, the radiation therapy apparatus includes a collimator device formed of a material the nature of which renders it impermeable to radiation such as tungsten or the like, thereby allowing the exposure to radiation to be limited to a therapy part including a body. Such a collimator device needs to have a function of carefully forming the irradiation field that approximates a shape of the therapy part without a formation of a penumbra. Accordingly, such a collimator device has a first collimator and a second collimator arranged in the irradiation direction such that they overlap.
With such an arrangement, the first collimator provided on a near side of a radiation source is configured in a form of a single unit comprising a pair of members disposed such that they face each other across an irradiation axis. Such an arrangement allows that the members drive so as to adjust a distance therebetween. For example, the members drive along an arc-shaped path around the radiation source as a center. On the other hand, the second collimator provided on a far side of the radiation source is configured in the form of a pair of collimator components (blocks) such that the collimator components face each other across the irradiation axis in an orthogonal direction to a moving direction of the first collimator. Each of the collimator components of the second collimator has multiple leaves arranged close to one another, which can be individually moved so as to adjust a distance therebetween along the arc-shaped path around the radiation source as the center.
The second collimator is called “multi-leaf collimator”, which includes, for example, a block pair composed of several ten leaves that are closely gathered side by side. The leaves of the second collimator each have the arc-shaped path and are individually driven movably along the arc-shaped path with a driving device provided to each leaf. Thus, the radiation field of an irregular shape can be formed by combining an operation of moving the first collimator composed of a pair of opposing separate members to get close to/away from each other in an X direction and an operation of individually moving the leaves of the second collimator, which are gathered opposite to each other, so as to get close to/away from each other in a Y direction. Here, the first collimator and the second collimator may be collectively referred to as “multi-leaf collimator”.
FIG. 15 schematically is one leaf 41 of the second collimator and a driving device 43 for driving the leaf. Referring to FIG. 15, how to drive the leaf 41 and control its position is described below in brief.
The leaf 41 of FIG. 15 is formed in a fan shape that converges to a radiation source as seen in a plan view and in a plate-like or wedge shape as seen in a side view. Further, an outer edge 41a of the leaf 41 is curved along the arc-shaped path around the radiation source as the center. A gear tooth is cut in the arc-shaped path plane, that is, the curved outer edge 41a. A driving gear 43a is engaged with the tooth of the outer edge 41a. 
The driving gear 43a is fixed to a tip end of a shaft 43b. The shaft 43b is driven based on a driving force transmitted from a motor 43c as a driving source through a driving force transmitting mechanism such as a worm gear 43d. A potentiometer 43e and an encoder 43f are arranged to detect a torque, and function as a position detecting device for the leaf 41. The motor 43c is controlled by an additional control device based on information from the potentiometer 43e and the encoder 43f, and the leaf 41 is set in a desired position to form a predetermined radiation field (see “Japanese Patent Publication (Laid-open: KOKAI) No. 2002-253686”, for example).
In the case of detecting penumbra, leaves of the multi-leaf collimator can be moved closer to/away from each other in a horizontal direction without requiring a complicated structure such as moves the leaves concentrically along the arc-shaped path around the radiation source as the center. One known technique applicable to this case is to drive a threaded shaft connected to each leaf with a motor through a flexible cable (see “Japanese Patent No. 2543373”, for example).
As described above, the technique disclosed in “Japanese Patent Publication No. 2002-253686” requires a mechanism for transmitting a torque of a motor to each of leaves of the second collimator through various gears and a shaft in order to individually move the leaves. Such a driving force transmitting mechanism needs to be provided to each leaf. From the viewpoint of radioprotection, in recent years, there is an increasing demand to further match a radiation field to an irregular shape of a treated area. To meet the demand, it has been examined whether to increase the number of leaves.
However, if the number of leaves is increased, it is necessary to prepare as many driving force transmitting mechanisms such as the motor, the gear and the shaft, as the number of leaves. This causes a problem about how to secure an area enough to arrange as many driving force transmitting mechanisms as the number of leaves in a limited space. Thus, increasing the number of leaves leads to a problem of enlarging the collimator to limit a treatment space of a radiation therapy apparatus itself.
Further, in the above driving force transmitting mechanism, a torque of the motor is transmitted to each leaf through the gear and the shaft, which imposes a limitation that the motor should be placed in a direction parallel or vertical to a moving axis of each leaf. Thus, the degree of freedom of arrangement is low. As a result, it is difficult to increase the number of leaves. Further, accumulative backlash between engaged gears causes an error in a moving amount of leaves and thus, it is necessary to take a measure for eliminating the backlash.
On the other hand, the technique disclosed in “Japanese Patent No. 2543373” transmits a torque of the motor to the threaded shaft connected to each leaf through a flexible cable to move the leaves by pushing or pulling in a horizontal direction. In the case of moving leaves along the arc-shaped path according to this technique, the flexible cable extends or contracts, resulting in a problem that the moving of each leaf cannot be managed with accuracy.